Day and Night
وَجَعَلْنَا اللَّيْلَ وَالنَّهَارَ آيَتَيْنِ ۖ فَمَحَوْنَا آيَةَ اللَّيْلِ وَجَعَلْنَا آيَةَ النَّهَارِ مُبْصِرَةً لِتَبْتَغُوا فَضْلًا مِنْ رَبِّكُمْ وَلِتَعْلَمُوا عَدَدَ السِّنِينَ وَالْحِسَابَ ۚ وَكُلَّ شَيْءٍ فَصَّلْنَاهُ تَفْصِيلًا (12)
 And We have appointed the night and the day as two Ayât (signs etc.). Then, We have obliterated the sign of the night (with darkness) while We have made the sign of day illuminating, that you may seek bounty from your Lord, and that you may know the number of the years and the reckoning. And We have explained everything (in detail) with full explanation.
There are 24 hours in a day
The day is divided into "day(time)" and "night(-time)". Daytime is from sunrise (this varies, but we can say approximately 6am) to sunset (we can say approximately 6pm). Night-time is from sunset to sunrise.
Every day starts precisely at midnight.
AM (Ante-Meridiem = before noon) starts just after midnight. PM (Post-Meridiem=after noon) starts just after noon. This means that 12am and 12pm have no meaning.